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Abstract: It’s All Black Shale: Relating Physical Scales and
Measured Values to Organic-rich Mudrocks
Marathon Oil Company Houston, TX
Organic-rich mudrocks are commonly described as black shales. They contain mixtures of various alternating microfabrics and compositions. Their sedimentary textures, structures, and mineral compositions can vary on a microscopic scale. Production variations from mudrock reservoirs occur on a much larger scale. Because of the multiple physical scales involved, it is a difficult task to relate the core analysis values of these mudrocks to petrophysical log and production values.
Many mudrock microfabrics are observed to be only a few millimeters thick. The laboratory analysis from a single 1-inch core plug can sample several individual microlithologies. Petrophysical logs generally represent rock properties from a scale 20 times greater than from a single core plug. Production volumes from a mudrock reservoir in a single well involve sampling from a scale several orders of magnitude greater than from petrophysical logs.
It is important to make meaningful relationships in measured rock properties across multiple physical scales. Laboratory improvements could include utilization of higher sampling frequencies. Petrophysical enhancements could include the utilization of tools with higher-frequency resolution. Bulk sampling of mudrocks might be advantageous when working with hydrocarbon composition data. The distribution of the critical data of importance will influence the sampling methodology which can best characterize the rock.